Polysulfide

Polysulfides are a class of chemical compounds containing chains of sulfur atoms. In their simplest form, polysulfides are anions with the general formula S_"n"²⁻ and the structure ⁻SS_n-2S⁻. These anions are the conjugate bases of the hypothetical acids H₂S_n.

Polysulfide salts

Although elemental sulfur is completely insoluble in water, a solution results upon addition sodium sulfide::Na₂S + 4 S → Na₂S₅:Na₂S + 5 S → Na₂S₆:Na₂S + 6 S → Na₂S₇:Na₂S + 7 S → Na₂S₈Alkali metals other than Na⁺ can be used. In some cases, these anions have been obtained as organic, which are soluble in organic solvents [cite journal | author = Dev, S.; Ramli, E.; Rauchfuss, T. B.; Wilson, S. R. | title = Synthesis and Structure of [M("N"-Methylimidazole)₆] S₈: Polysulfide Salts Prepared by the Reaction N-Methylimidazole + Metal Powder + Sulfur | journal = Inorg. Chem. | year = 1991 | volume = 30 | pages = 2514 | doi=10.1021/ic00011a011]

A useful polysulfide is lime sulfur CaS_n, which is used as a pesticide in agriculture.

Applications

Polysulfide polymers

In the commercial world polysulfide usually refers to a class of polymers with alternating chains of several sulfur atoms and hydrocarbons. The general formula for the repeat unit is – [(CH₂)_m-S_x] _n– , where x indicates the number of sulfur atoms (or rank), and n indicates the number of repeat units. Polymers containing sulfur atoms separated by hydrocarbon sequences are not polysulfides, e.g. polyphenylene sulfide (C₆H₄S)_n.

Polysulfide polymers can be synthesized by condensation polymerization reactions between organic dihalides and alkal metal salts of polysulfide anions::n Na₂S₅ + n ClCH₂CH₂Cl → [CH₂CH₂S₅] _n + 2n NaCl

Dihalides used in this condensation polymerization are dichloroalkanes (such as 1,2-dichloroethane, bis-(2-chloroethyl)formal (ClCH₂CH₂OCH₂OCH₂CH₂Cl), and 1,3-dichloropropane. In some cases, polysulfide polymers can be formed by ring-opening polymerization reactions.

Polysulfide polymers are insoluble in water, oils, and many other organic solvents. Because of their solvent resistance, these materials find use as sealants to fill the joints in pavement, automotive window glass, and aircraft structures.

Polysulfides in vulcanized rubber

Many commercial elastomers contain polysulfides as crosslinks. These crosslinks interconnect neighboring polymer chains, thereby conferring rigidity. The degree of rigidity is related to the number of crosslinks. Elastomers therefore have a characteristic ability to "snap back" to their original shape after being stretched or compressed. Because of this memory for their original cured shape, elastomers are commonly referred to as rubbers. The process of crosslinking the polymer chains in these polymers with sulfur is called vulcanization. The sulfur chains attach themselves to the "allylic" carbon atoms, which adjacent to C=C linkages. Vulcanization is applied to the processing of several classes of rubbers, including polychloroprene (Neoprene^TM), styrene-butadiene, and polyisoprene, which is natural rubber. Charles Goodyear's discovery of vulcanization, involving the heating of polyisoprene with sulfur, was revolutionary because it converted a sticky and almost useless material into an elastomer which could be fabricated into useful products.

Energy storage

The energy released in the reaction of sodium and elemental sulfur is the basis of a battery. Sodium-sulfur batteries require high temperatures to maintain liquid polysulfide and Na⁺-conductive membranes that are unreactive toward sodium, sulfur, and sodium sulfide.

Polysulfido complexes

Polysulfides are common ligands in coordination chemistry. Example of transition metal polysulfido complexes include (C₅H₅)₂TiS₅, [Ni(S₄)₂] ²⁻, and [Pt(S₅)₃] ²⁻. [Draganjac, M. E.; Rauchfuss, T. B., "Transition Metal Polysulfides: Coordination Compounds with Purely Inorganic Chelate Ligands", Angewandte Chemie International, Edition in English, 1985, vol. 24, 742.] Main group elements also form polysulfides. [Takeda, N.; Tokitoh, N. and Okazaki, R., "Polysulfido Complexes of Main Group and Transition Metals", Topics in Current Chemistry, 2003, vol. 231, 153-202. ISBN 3540403787.]

References